Police in Texas say they found eight people dead early Sunday in the back of a sweltering tractor-trailer in a Walmart parking lot. A ninth victim died in a hospital hours later.

Surveillance footage recorded vehicles pulling up to the truck Saturday night, taking a few people at a time from the trailer, then closing the truck and driving away.

The truck's human cargo was discovered after someone from the trailer asked a Walmart employee for water. That led to a police check, and the discovery of about 30 survivors, including two school-aged children, along with those who died - all adult men. Authorities at a hospital where the survivors were taken said 20 people were in critical condition.

"We're looking at a human trafficking crime this evening," San Antonio police chief William McManus said, calling the case a "horrific tragedy."

San Antonio's fire chief, Charles Hood, said it was likely no one would have survived another night in the trailer, after a day in which temperatures reached 38 degrees Celsius.

The trailer was a refrigerated freight carrier, but its cooling system was not working.

A 60-year-old man from Florida, said to be the truck's driver, was arrested, and authorities said he would face criminal charges in court on Monday. Homeland Security Secretary John Kelly promised a thorough investigation.

"These smugglers have no regard for human life and seek only profits," Secretary Kelly said in a statement Sunday. He pledged that authorities in the U.S., Mexico and Central America will "root out these smugglers, bring them to justice and dismantle their networks."

There was no word on the nationality of the immigrants aboard the truck. San Antonio is a few hours' drive from the U.S.-Mexico border, but people smugglers who bring in illegal immigrants are patronized by people from a number of Central American countries as well as Mexico.

The Mexican government said in a statement it deeply regrets the deaths, and that it has called on authorities to conduct an exhaustive investigation.